Hemolysis Types Blood Agar
Beta Hemolysis
Streptococcus pyogenes, or Group A beta-hemolytic Streptococci (GAS), and Streptococcusagalactiae, or Group B beta-hemolytic Streptococci (GBS) blood agar cultures display beta
hemolysis. Beta hemolysis (β-hemolysis), sometimes called complete
hemolysis, is a complete lysis of red cells in the media around and under the colonies: the area
appears lightened (yellow) and transparent.
Streptolysin, an exotoxin, is produced by the bacteria which causes the complete lysis of red
blood cells. Streptolysin O is oxygen-sensitive cytotoxin, secreted by most GAS, and interacts
with cholesterol in the membrane of eukaryotic cells (mainly red and white blood cells,
macrophages, and platelets), and usually results in β-hemolysis under the surface of blood agar. Colonies of group B streptococci often have less pronounced zones of beta-hemolysis
than do other beta-hemolytic streptococci.
Alpha Hemolysis
Streptococcus pneumoniae, Streptococcus salivarius, viridans are referred to collectively as
viridans streptococci, a name derived from viridis (Latin for "green"), referring to the green
pigment formed by the partial, α-hemolysis of blood agar. Encapsulated, virulent strains of S.
pneumoniae often forming highly mucoid, glistening colonies (production of capsular
polysaccharide) surrounded by a zone of α -hemolysis.
When α-hemolysis is present, the agar under the colony is dark and greenish. Streptococcus
pneumoniae and a group of oral streptococci (Streptococcus viridans or viridans streptococci)
display alpha hemolysis. This is sometimes called green hemolysis because of the color
change in the agar.
Other synonymous terms are incomplete hemolysis and partial hemolysis. Alpha hemolysis is
caused by hydrogen peroxide produced by the bacterium, oxidizing hemoglobin to green
methemoglobin.
Alpha-hemolytic colonies with depressions in their centers are characteristic of pneumococci. Some strains produce high ammounts of capsular polysaccharide which gives
glistening appearance.
Gamma Hemolysis
If an organism does not induce hemolysis, the agar under and around the colony is
unchanged, and the organism is called non-hemolytic or said to display gamma hemolysis (γhemolysis). Enterococcus faecalis (formerly called Group D Streptococci) displays gamma
hemolysis.
E. faecalis typically exhibits gamma-hemolysis on blood agar, but some strains are alfahemolytic or even beta-hemolytic (a plasmid-encoded hemolysin, called the cytolysin).
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